hedwards: I think quake was the last game that I played like that. By the time that Quake 2 came out, you pretty much had to use mouse + keyboard for singleplayer games, and I think with Quake you had to use the combo if you wanted to play multiplayer competitively.
timppu: You played Quake with keyboard only? Wow... :)
I made the move to mouse + keyboard with Duke3D. I first tried to play it like I had always played Doom 1-2, with the keyboard-only setup mentioned earlier, but it felt quite cumbersome as you also needed to look a bit of up/down. I tried mouse once for mouselook... whoa! I instantly started playing Duke3D 4x better. Naturally I played all FPS games after Duke3D also with mouse+kb.
With Doom 1-2 (single-player) mouse was not as important as there was not look up and down, the autoaim in Doom was very heavy, and at least with the default setup the mouse had the irritating feature that it would move you forwards/backwards, which was very disorientating if you were already using the keyboard for that. Maybe there was a way to disable that.
I think that Quake was the last FPS I've played that more or less gimped the keyboard + mouse control. I never really saw the point to it as the game autoaimed vertically. It wasn't until you needed to hit something fairly far off quickly with something other than the shot gun that you really needed the mouse.
I guess I should take it back, I remember playing Unreal that way as well.
These days though, I use mouse as well, but at the time I'd invested about 6 years of playtime with keyboard and mouse and I didn't like having to relearn all that.
But, you know, time makes fools of us all.